Method of producing a hot mix, cold laid paving mixture



Patented Jan. 4, 1938 UNITED STATES PATENT QFFECE METHOD OF PRODUCING AHOT MIX, COILD LAID PAVING MIXTURE No Drawing. Application August 30,1934, Serial No. 742,148

2 Claims.

This invention relates to the manufacture of bituminous pavements,particularly to a method in which high softening point and lowsusceptibility to temperature change asphalts are introduced in densepaving mixtures.

The general method used in preparing hot mix pavements is by preheatingmineral aggregate to approximately 375 F. and coating this aggregatewith asphalt ranging in softening point from 110 F. up to 140 F. Thisasphalt is also preheated to approximately 325 F. so that it willeffectively spread over the mineral particle surfaces. The practicaldimculties encountered with this general method are:

(1) The requirement of high mixing temperatures and large equipment suchas kilns 24 feet long by feet in diameter, steam boilers to liquefy thepaving asphalt which is usually shipped in tank cars, etc.

(2) lhe necessity of spreading out such mixtures on the road surfacewith the least time interval between the mixing and laying andimmediately compacting same while hot.

(3) The inability to produce paving mixtures by the standard hot mixmethods that will have a high stability to distortion under loads in hotweather and will not be inordinately brittle at low temperatures.

It is possible to produce high melting point paving asphalts (170 to 200F. softening point) which would have approximately the same penetrationor hardness at 77 F. as standard paving asphalts and yet haveconsiderably higher penetration or be perceptibly softer at 0 F. The useof such high melting point low susceptibility to temperature changeasphalt is restricted by virtue of the fact that it would requirepreheating the mineral aggregate to at least 425 F. and the asphalt toabout 400 F. in order to obtain a homogeneous mixture. Furthermore,paving mixtures of such high temperatures would chill too rapidly andwould not allow for sufficient transportation interval from mixers toroad surfaces without having to constantly move the plant every mile ortwo. This difl'iculty is largely due to the fact that the lower thesusceptibility of asphalt to temperature change, the higher itsviscosity at elevated temperatures as compared to similar melting pointasphalts that are high in susceptibility to temperature change.

An object of this invention is to manufacture bituminous pavements inwhich the asphalt used has a high softening point.

Another object of this invention is the manufacture of bituminouspavements in which the asphalt used has a low susceptibility totemperature change and thereby high stability to distortion under loadsat summer temperatures.

According to this invention, the mineral aggregate is preheated to 150to 350 F. This may be done in a drum 12 feet long, 4 feet in diameter,and mounted on a truck trailer so that it may be easily transported. Theslightly preheated aggregate is then mixed with a priming oil or softbitumen ranging in consistency from 300 seconds viscosity furol at 122F. to 70 F. softening point, with or without preheating. At the sametime powdered asphalt, having a softening point above 250 F., isintroduced in the right proportion to obtain an asphalt having thedesired melting point upon amalgamation with the priming oil or softbitumen that is used. By this method a paving mixture is obtained inwhich the asphalt formed by amalgamation may be of any desired softeningpoint according to the proportions of the priming oil or soft bitumenthat is used with the powdered asphalt.

An advantage obtained in preparing a paving mixture of this type is thatthe priming oil or soft bitumen, together with the powdered asphalt, isonly partially amalgamated when first mixed and may be laid hot orallowed to cool or partially cool before laying. Upon rolling andtraffic the amalgamation of the priming oil or soft bitumen with thehard powdered asphalt is accomplished.

Inasmuch as the above description has been given merely for the sake ofillustration and not for the purpose of limiting the invention thereto,it is intended to claim all inherent novelty in the invention as broadlyas the prior art permits.

I claim:

1. A method of producing a paving mixture containing as a binder anasphaltic material having a softening point of 170 to 200 F. and adaptedto be laid and compacted while cool by application of pressure, whichcomprises heating a mineral aggregate to a temperature of 150 to 350 F.and mixing therewith a preheated soft bitumen ranging in consistencyfrom 300 seconds viscosity furol at 122 F. to 70 F. softening point anda powdered asphalt.

2. A method of producing a paving mixture containing as a binder anasphaltic material of 170 to 200 F. softening point, adapted to be laidand compacted while cool by application of pressure, which comprisesheating a mineral aggregate to a temperature of 150 to 300 F. and mixingtherewith a preheated soft bitumen ranging in consistency from 300seconds viscosity furol at 122 F. to 70 F. softening point and apowdered asphalt having a softening point above 250 F.

CHARLES M. BASKIN.

